1. Field of the Invention
The invention relates to a shift control apparatus and shift control method for a vehicle. More particularly, the invention relates to upshift control during high acceleration, that is, when a request for substantially maximum acceleration is made by a driver.
2. Description of the Related Art
A high acceleration time shift control apparatus for a vehicle is known which includes (a) a transmission that achieves plural shift speeds whose gear ratios are different from each other, and (b) high acceleration time upshifting means for changing the shift speed of the transmission to a higher speed based on a predetermined rotational speed for determination (hereinafter, referred to as a “determination rotational speed”) such as a vehicle speed, such that an input rotational speed of the transmission substantially reaches a predetermined target maximum rotational speed when a request for high acceleration is made by a driver, for example, when an accelerator pedal operation amount is large. An example of the above-mentioned apparatus is disclosed in Japanese Patent Laid-Open Publication No. 10-089455. The high acceleration time upshifting means includes (c) shift determining means for outputting an upshift command for performing an upshift when the determination rotational speed reaches a predetermined speed at which it is determined that shifting can be performed (hereinafter, referred to as a “shift determination speed”), and (d) ineffective time calculating means for obtaining an actual ineffective time until shifting is actually started and the input rotational speed starts decreasing after the upshift command is output. The high acceleration time upshifting means (e) successively calculates a rate of change of the input rotational speed of the transmission during actual acceleration, and sets the shift determination speed such that the input rotational speed reaches the target maximum rotational speed in the ineffective time when the input rotational speed changes at the rate of change.
However, the shift determination speed is set based on the rate of change of the input rotational speed during actual acceleration, on the precondition that the input rotational speed changes at the same rate of change even after the upshift command is output. Therefore, a change (decrease) in the rate of change, for example, due to a torque phase is not reflected. Also, the rate of change (hereinafter, referred to as the “change rate”) changes due to a disturbance such as a change in a road inclination. Accordingly, it is not always possible to perform shifting at a rotational speed substantially equal to the target maximum rotational speed with high accuracy. Also, there is another problem that, since the shift determination speed is set according to the actual change rate each time upshifting is performed during high acceleration, the shift determination speed changes each time upshifting is performed, and the control become unstable.